1. Field of the Invention
The present invention relates to a thermal transfer line printer, and particularly, to a small-sized thermal transfer line printer that can reciprocate a recording medium to form a full color image on the surface of the recording medium.
2. Description of the Related Art
Conventionally, a thermal transfer line printer that can perform recording in a direction orthogonal to the conveying direction of a recording medium by means of a line thermal head having a length corresponding to the recording ranges of a recording medium in its longitudinal and transverse directions is known (for example, refer to JP-A-08-072335 (FIG. 1)).
FIG. 11 shows an example of a conventional thermal transfer line printer. This conventional thermal transfer line printer 101 is configured so as to be able to perform recording in a direction orthogonal to the conveying direction of a recording medium 104 by turning down a line thermal head 105 having a length corresponding to the recording range of a recording medium 107 in its longitudinal or transverse direction and making the line thermal head abut against a platen roller 102 by way of an ink film 103, such as an ink ribbon or an ink sheet, or a recording medium 104, such as a recording sheet, rotationally driving the platen roller 102, and a first conveying roller 106 and the second conveying roller 107 that constitute a conveying mechanism in a state where the line thermal head 105 that is turned down is abutted against the platen roller 102, and selectively driving a plurality of heat generating elements of the line thermal head 105 on the basis of recording information while the ink film 103 and the recording medium 104 are conveyed to make them generate heat, thereby thermally transferring the ink of the ink film 103 to the recording medium 104.
More specifically, when recording is performed by the thermal transfer line printer 101, the recording medium 104 is conveyed towards the right of FIG. 11 from the left of FIG. 11 by a conveying mechanism made up of the first conveying roller 106, the second conveying roller 107, etc.
The first conveying roller 106 and the second conveying roller 107 are disposed on the right and left of the line thermal head 105, and a first pressure contact roller 106a and a second pressure contact roller 107a that are brought into pressure contact with the first conveying roller 106 and the second conveying roller 107 to rotate following them are disposed above the first conveying roller 106 and the second conveying roller 107. Also, the recording medium 104 can be sandwiched by the first second conveying roller 106 and 107 and the first and second pressure contact roller 106a and 107a. As the conveying rollers 106 and 107 rotate in the forward rotation direction or reverse rotation direction, for example, in the clockwise direction, or counterclockwise direction, the recording medium 104 can be conveyed in the right and left directions. Specifically, the forward conveyance of a recording medium from the upstream side to the downstream side in the conveying direction that is directed to the right of FIG. 11, and the reverse conveyance of a recording medium from the downstream side to the upstream side in the conveying direction that is directed to the left of FIG. 11, can be performed.
On the other hand, the platen roller 102 that is rotated in the forward rotation direction or reverse rotation direction (for example, in the clockwise direction or counterclockwise direction) is disposed in a lower part of FIG. 11 that faces the line thermal head 105, and the recording medium 104 can be sandwiched between the line thermal head 105 and the platen roller 102.
The first conveying roller 106 and the second conveying roller 107 are rotationally driven by sequentially transmitting the driving force of a driving motor (a first driving motor) (not shown) that is provided below the first conveying roller 106.
That is, the driving force of the first driving motor is transmitted to one end of a rotary shaft of the first conveying roller 106, thereby rotationally driving the first conveying roller 106, and a driving force is transmitted to the second conveying roller 107 from the other end of the first conveying roller 106 to which that the driving force of this first driving motor has been transmitted, thereby rotationally driving the second conveying roller 107. Generally, toothed belt transmission and gear transmission are used for such driving force transmission.
As for the rotational driving of the platen roller 102, a driving motor (a second driving motor) (not shown) is separately provided below the platen roller 102, and the platen roller 102 is rotationally driven independently by the driving force of the driving motor.
Generally, an ink film having a configuration in which three color inks of at least three primary colors including Y (yellow), M (Magenta), and C (cyan) are repeatedly arranged in the conveying direction of the recording medium 104 in is used for full color recording.
When full color recording is performed in the thermal transfer line printer 101 having such a configuration, the head of the recording medium 104 is first detected by a medium detection sensor 108 in first color recording operation (first recording operation). At that time, the recording medium 104 is sandwiched by the first conveying roller 106 and the first pressure contact roller 106a. Thereafter, when the recording medium 104 is conveyed to the downstream side in the right and left conveying direction of FIG. 11 by the first conveying roller 106 (forward conveyance), a front end of the recording medium 104 will be sandwiched between the line thermal head 105 that is turned down and the platen roller 102.
At this time, the recording medium 104 is sandwiched at two places by the pressure contact between the first conveying roller 106 and the first pressure contact roller 106a, and the pressure contact between the line thermal head 105 and the platen roller 102.
Then, while the recording medium 104 is sandwiched between the platen roller 102 and the line thermal head 107, the recording medium 104 is conveyed to the downstream side, and thermal transfer of the first color ink is started from the front end of the recording medium 104. In the course of this thermal transfer, the recording medium 104 is sandwiched at three places by the pressure contact between the first conveying roller 106 and the first pressure contact roller 106a, the pressure contact between the line thermal head 105 and the platen roller 102, and the pressure contact between the second conveying roller 107 and the second pressure contact roller 107a. 
When the first recording operation is completed, the line thermal head 105 is turned up against the biasing force of a spring 109. Then, the recording-medium 104 that is brought pressure contact with and sandwiched between the second conveying roller 107 and the second pressure contact roller 107a and has been subjected to the first color recording is conveyed in the reverse direction (reverse conveyance) towards the upstream side in the conveying direction in the left direction of FIG. 11 between the line thermal head 105 that is turned up and the platen roller 102, by rotational driving in the counterclockwise direction (reverse rotation direction) of the second conveying roller 107.
Then, the reversely conveyed recording-medium 104 pushes down a contact 108a of the medium detection sensor 108 leftward of FIG. 11. Further, the recording medium 104 is sandwiched by the second conveying roller 107 and the second pressure contact roller 107a, and is further fed back by counterclockwise rotation of the first conveying roller 106.
Thereafter, when the contact 108a of the medium detection sensor 108 is out of the front end of the recording medium 104 and becomes upright, the front end of the recording medium 104 is detected, and then the rotation of the first conveying roller 106 is stopped. Then, the same recording operation as the first recording operation is repeated, thereby overlappingly recording an image of a second color on the image of the first color in the second recording operation.
Then, the same process is performed, thereby overlapping recording images of third or fourth colors on the image of the second color, so that a desired color image can be recorded on the recording medium 104 in the third and fourth recording operation.
However, in the conventional thermal transfer line printer 101, the driving force of the first driving motor is transmitted to the first conveying roller 106, and the driving force is re-transmitted to the second conveying roller 107 from the first conveying roller 106 to which the driving force has been transmitted, and a rotation driving mechanism in which backlash is provided in meshing portions between teeth for toothed belt transmission, gear transmission, etc. is provided are used for the transmission of the driving forces. Therefore, the total amount of backlash in a transmission path of a driving force is obtained by adding individual backlashes. Thus, there is a problem in that, as the total number of meshing portions between teeth interposed between a driving member, such as a motor, and driven members, such as the conveying rollers 106 and 107, becomes more, the total amount of the backlash become large, consequently uneven conveyance of the recording medium 104 occurs, and thus exact conveyance cannot be performed. As a result, when full color recording is performed on the recording medium 104, there is also a fear that color deviation is caused in different ink colors on an ink film 103 where overlapping recording is made, and thus high-quality recording cannot be performed.
Further, in the conventional thermal transfer line printer 101, the driving force of the first driving motor is transmitted to the first conveying roller 106, and the driving force is re-transmitted to the second conveying roller 107 from the first conveying roller 106 by which the driving force has been transmitted. Therefore, there is also a problem in that deviation may be caused between the starting timing of the first conveying roller and the starting timing of the second conveying roller 107 due to a difference in the amount of backlash.
In addition, in the conventional thermal transfer line printer 101, there is also a problem in that the first driving motor that drives the first and second conveying rollers 106 and 107, and the second driving motor that drives the platen roller 102 are provided independently, and a need for reducing cost cannot be met.
Thus, a printer that can record high-quality recording without causing conveyance unevenness of a recording medium nearly is required.